Charged-particle-beam (CPB) image-transfer and exposure apparatus, such as electron-beam reduction image-transfer devices and the like, have recently been developed. CPB image-transfer and exposure apparatus provide improved productivity and resolution of transferred patterns relative to light-based image-transfer and exposure devices.
It is desirable that CPB image-transfer devices have as large an optical field as possible, that is, the pattern area that can be transferred at once as a whole via the projection system of the device should be as large as possible. But, when the optical field of such a projection system is enlarged, it becomes difficult to keep imaging characteristics (such as focal-point position, image rotation, magnification, and the like) within prescribed permissible ranges over the entire optical field. For this reason, correction lenses are typically included in such a projection system for the purpose of correcting the imaging characteristics.
FIG. 1 illustrates a prior-art electron-beam image-transfer device (electron-beam exposure apparatus) equipped with correction lenses for the projection system. In FIG. 1, an electron beam EB passes through a pattern field on a reticle 3. Although not shown in FIG. 1, the electron beam EB upstream of the reticle 3 is emitted from an illumination system comprising an electron beam source, an illumination lens, a deflector controlling the illumination field of the electron beam, and astigmatism-correction device. After passing through the reticle, the electron beam EB forms an image of a reticle pattern on a wafer 10 or other substrate via a projection system P comprising a first projection lens 7 and a second projection lens 8. Positioned between the first projection lens 7 and the second projection lens 8 are a first rotation-correction lens 31 and a second rotation-correction lens 32.
The projection lenses 7 and 8 are electromagnetic lenses. The projection lenses 7 and 8 rotate the pattern image as it is transferred to the wafer 10, due to the rotational effects of the magnetic field produced by the projection lenses. The projection lenses 7 and 8 are typically designed to rotate the image so that the orientation of the image on the wafer 10 will coincide with a prescribed orientation.
In practice, the image rotation frequently does not exactly meet specification due to design error in the projection lenses 7 and 8, manufacturing error, height error of the stage on which the wafer 10 is supported, and the like. The exact image rotation thus must be controlled by the two rotation-correction lenses 31 and 32.
Image rotation is known to be generally proportional to the integral value of the on-axis magnetic field. For this reason, the sum of the integral value of the on-axis magnetic field of the first rotation-correction lens 31 and that of the second rotation-correction lens 32 is proportional to the correction of image rotation.
One difficulty with correction of image rotation is that the focal-point position of the image changes significantly when the magnetic fields of the rotation-correction lenses 31, 32 are aligned, even though a large image rotation can be achieved with such alignment. Hence, the rotation-correction lenses 31, 32 are typically driven with currents, the directions of which are chosen such that the generated magnetic fields are in opposite directions, in order to reduce the change in the focal-point position.
The two rotation-correction lenses 31, 32 are typically positioned so as to correct the image rotation of the image and so as to set the magnetic fields of the two rotation-correction lenses 31, 32 in opposite directions. As a result, the focal-point position of the image is not displaced significantly by the rotation-correction lenses 31, 32. But, when the image rotation is corrected while minimizing any change of the focal-point position by setting the magnetic fields of the two rotation-correction lenses 31, 32 in opposite directions, the magnification of the transferred image is adversely affected. And, when the two rotation-correction lenses 31, 32 are driven so as to eliminate adverse changes in magnification, the focal-point position of the image is displaced significantly.
Independently employing an image-rotation-correction lens and a focal-point position-correction lens has been proposed for the purpose of correcting the image rotation and the focal-point position, respectively. With such a scheme, the focal-point displacement generated by the image-rotation-correction lens can be adjusted by using the focal-point-correction lens. But, with such a scheme, image rotation changes whenever the focal-point-correction lens is employed. This makes it difficult to keep the imaging characteristics as a whole within a prescribed permissible range.